This invention relates to apparatus for transporting heat efficiently by means of a liquid/vapor or two-phase transport system, and more particularly to apparatus of this type in which the liquid is transported or distributed bi-dimensionally along separate and intersecting distribution channels, the distribution channels in one direction extending through porous wicks disposed in respective vapor distribution channels, the apparatus providing a low pressure loss geometric configuration which in conjunction with the substantial pressure head permitted by the wick and the large surface area available provides very high heat transport capacity.
Thermal control in, for example, spacecraft has been provided through the use of several types of heat transport systems, these being either an active control system or a passive control system. An active thermal control system requires some type of fluid being pumped through the components of the system. In spacecraft of the prior art such fluid systems were of the single phase heat transport types wherein a liquid is heated as it passes through the heat sources and rises in temperature, and thereafter gives up heat in a heat sink, such as a spacecraft radiator, and drops in temperature. For a large system with high heat loads the liquid flow rate must be substantial, and such large flow rates require large pumps which, of course, utilize large amounts of electrical power which is a critical resource of spacecraft systems. This is a major deficiency of the single phase active thermal control system, but additionally, the liquid will generally also have a large temperature gradient between the heat sources and the heat sink. One of the advantages of this system is that it can operate under gravitational forces in addition to the near zero gravity environment of a low earth orbit.
Other thermal control systems developed for spacecraft utilize a passive system theory. For example, a heat pipe utilizes the latent heats of vaporization and condensation of the liquids and vapors. A typical heat pipe has a circular cross sectional configuration and along its length has an evaporator section and a condenser section separated by a substantially adiabatic section. A porous capillary wick is disposed within the pipe intermediate the axis and the body thereof. The vapor flows through the central portion of the pipe in one direction from the evaporator to the condenser while liquid flows in the opposite direction by the action of the capillary forces created by the wick. Heat is added in the evaporator section of the pipe which causes liquid contained within the porous wick to evaporate. The vapor, due to a locally high vapor pressure, flows through the pipe toward the condenser section where heat is removed and the vapor condenses in the wick material. The liquid in the wick material is transported by the capillary forces associated with the wick toward the evaporator section. Since the liquid evaporates and the vapor condenses at substantially the same temperature, very small temperature gradients exist between the heat source and the heat sink. Additionally, since the latent heat of vaporization for most fluids is large, very small mass flow rates are required to transport significant amounts of heat from the source to the sink. Moreover, since the mass transport occurs passively due to the action of capillary forces, no electrical energy is required to operate the heat pipe. However, although they are very efficient devices and are quite effective in the microgravity environment of low earth orbit where capillary forces can predominate, heat pipes are ineffective under the gravitational forces on earth where the small capillary forces cannot predominate.
In an effort to overcome the capillary pressure limitations of heat pipe systems, yet retain the inherent advantages of two-phase transport systems, several concepts are presently under study for future systems. One such concept uses a pump on the liquid side of the system for pumping the liquid to the heat sources, the liquid being metered through control valves prior to entry through the heat source evaporators. The control valves operate to meter the liquid so that it completely evaporates to vapor at the exit of the evaporators. The vapor then passes to the heat sink radiator elements where it condenses, and the liquid is then subcooled prior to entering the inlet of the pump. Although pumped two-phase thermal bus systems are envisioned as having high heat transport capacities with small power consumption, the process is no longer passive, as in a heat pipe, but must be actively designed, monitored and controlled. Thus, the major drawback appears to be in the complexity of the engineering technology required to ensure proper management of the liquid and vapor, especially in microgravity conditions.
Another concept for heat transport, known as a capillary pumped loop, which is described in NASA publication TM X-1310, Nov. 1966, utilizes a capillary device only in the evaporator. Heat is added to the evaporator and the vapor generated is forced to flow in one direction from the capillary pump, the vapor acting to force all the mass to flow through the system. Condensation occurs in the cooler sections of the loop and the liquid is pushed back to the inlet of the evaporator through a perforated conduit about which the wick is disposed. The liquid thereby wets the porous capillary plug and when heated is vaporized. Although the capillary pumped loop operates well under microgravity conditions, it also has limited ability for operating under gravitational forces. Additionally, the capillary pumped loop is sensitive to pressure loss in the condenser duct and the liquid returned to the evaporator must be slightly subcooled for operation to be maintained.
Thus, each of the known prior art heat transport systems for moving heat from heat sources to heat sinks for use in spacecraft has limitations which reduce their utility for such application. To summarize, the single phase, pumped liquid system requires a high power consuming pump; the passive heat pipe systems have limited heat transport capacity; the actively pumped two-phase thermal bus concept, although having large heat transport capacity, is overly complex; while the capillary pumped loop system, which although is passive and has improved capacity over heat pipes, is sensitive to pressure loss and subcooling. Additionally, in each of the two-phase systems the preferred working fluid is ammonia, which has high toxicity.